We fabricated a 50-nm-wide slit on a glass substrate using the lift-off process with a bilayer resist including Cr. The nanometric slit is designed for detecting a small number of neutral atoms in the ground state with high spatial accuracy by way of two-step photoionization with two-color near-field lights. Conducting the finite difference time domain simulations, we drew the spatial profiles of near-field lights generated in the vicinity of the slit for both cases of normal illumination and total-internal reflection (TIR) illumination with a linearly polarized light beam. In the slit-parallel-incident TIR configuration with s-polarization, we obtained a good throughput of 1.1 x 10-2 and the highest extinction ratio of 2.2 x 10-7 for a wavelength of 780 nm. In this case, the ionization efficiency of low-energy 87Rb atoms in the 5S1/2 hyperfine state is estimated to exceed 1%.